I'm building a website, and I want my Arduino Uno (that is hooked up by USB to my local machine) to execute every time a certain page is reached on the site. (For example, blink the light when somebody signs up for our service.) I don't have an Ethernet shield. Is there an alternative?

The only thing that I've come up with is to repeatedly download a log file from the website to my local machine, and run a loop querying the contents of that log file on my Arduino, but the constant requests will put too much stress on the server. Is there any easier way to do this?

2 Answers
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When the page is requested on the server, you can run a program that signals your local machine that is controlling the Arduino. Implemenaation depends on what Web Server you are running and if the content is static or not. But even if it just a simple static html page, that page can include a request to this service much like a html page includes requests for included images.
Your local machine will need a static IP address. If the local machine is on a different network behind a generic router, you may need to do port forwarding to the static address.

You need to use Processing to ping the website, and send the acquired data through the serial port to the Arduino. Since both are using the same port, make sure to load the code over to the Arduino first before you start running the processing code.